The present invention relates to a method for forming a resist pattern employed in a process of fabricating semiconductor devices, and in particular to photomasks used in such a method.
In photolithography by projection exposure, various methods have been proposed which can form fine resist patterns for coping with the packing density increase of semiconductor devices. One technique drawing attention is a phase-shifting mask method.
The phase-shifting mask method was reported, for example, in IEEE Transaction of Electron Devices, Vol. ED-29 (1982), p. 1828, and Vol. ED-31 (1984), p. 753, by Levenson, et al. In the phase-shifting mask method, a phase-shifter in the form of a transparent thin film for shifting the phase of light passing therethrough is disposed partially over the photomask. At the edge of the phase-shifter (the boundary between the area covered by the phase-shifter and the area not covered), the light passing through the phase-shifter and the light not passing through the phase-shifter, each 180.degree. out of phase from each other, interfere with each other, and the light intensity on the wafer is reduced, and the part of the resist along the edge of the phase-shifter is effectively unexposed to the light. By the use of the phase-shifter, the resolution of the projection exposure method is thereby improved.
An application of the phase-shifting mask method is disclosed in Japanese Patent Application No. 190162/1990 and in Paper No. 27p-ZG-3, Extended Abstracts (The 51st Autumn Meeting, 1990); The Japan Society of Applied Physics. This method will be described with reference to FIG. 3A, FIG. 3B and FIG. 4. The drawings show an example in which resist patterns for forming gate patterns having a gate electrode and a pad. FIG. 3A and FIG. 3B are plan views showing the photomasks used, and FIG. 4 is a plan view showing a latent image formed on the resist when the photomasks of FIG. 3A and FIG. 3B are used.
In the illustrated pattern formation method, the photoresist is exposed to light through a first photomask 10 having a transparent part 11, a rectangular opaque part 13 and a phase shifter 15 having part of its edges positioned in the transparent part 11 (FIG. 3A). Another exposure is made using a second photomask 20 having an opaque part 21 corresponding to the opaque part 13 of the first photomask 10 and an opaque part 25 corresponding to an edge of the phase shifter 15 positioned in the transparent part 11 (FIG. 3B).
By the exposure using the first photomask 10, the area of the resist covered by the opaque part 13 does not receive light, and the area along the edge lines of the shifter 15 does not receive light by virtue of the interference (i.e., by the effect of phase-shifter edge line mask) and these areas remain unexposed, while the rest of the area receives light.
By the exposure using the second photomask 20, the areas covered by the opaque parts 21 and 25 do not receive light, while the rest of the area receives light. When the two exposures have been effected, a part 31 which is covered by both of the opaque area 13 of the first photomask 10 and the opaque area 21 of the second photomask 20, and a part 33 which is along the edge lines of the shifter 15 of the first photomask 10, and also is covered by the opaque part 25 of the second photomask 20, remain unexposed, while the rest of the area receives light during the first exposure and/or the second exposure. As a result, a latent image, shown in FIG. 4, consisting of the unexposed parts 31 and the unexposed part 33 is formed.
Where the resist is of the negative type, when the latent image of FIG. 4 is developed, a resist pattern with the unexposed parts 31 and 33 having been removed is obtained. This resist pattern can be utilized for forming a gate electrode and a pad by the lift-off method. Where the resist is of the positive type, when the latent image of FIG. 4 is developed, a resist pattern consisting of the unexposed parts 31 and 33 is obtained. In this case too, the area corresponding to the unexposed part 33 will be a part for forming a gate electrode, and the area corresponding to the unexposed part 31 will be a part for forming a pad of the resist pattern.
With the pattern forming method described above, a resist pattern for forming a rectangular pad and a gate electrode having a line width (corresponding to the gate length) of 0.2 .mu.m or less can be formed with ease.
A problem associated with the above described pattern forming method is that if the second photomask 20 is not exactly in alignment but is offset relative to the first photomask 10, the area 31 for the pad, which is unexposed through the first and second exposures, is reduced. For instance, if the second photomask 20 is offset in the direction of the length of the gate (horizontal direction in the figure), by X0 as shown in FIG. 5A, the width of the resultant unexposed part 31a will be W1', smaller than the designed width W1 by X0. Moreover, if the second photomask 20 is offset to the right as seen in the figure, a part of the upper horizontal (as seen in the figure) edge of the shifter 15 which is designed to be exposed during the second exposure is not exposed by the area 41 near the right edge of the opaque part 21 (due to the offset X0 of the opaque part 21), so a spike-like unexposed part 15a results, as shown in FIG. 5B.
To avoid the size reduction of the rectangular part and the formation of the spike-like part 15a, the opaque part may be made larger than the designed dimension of the pad, as shown in FIG. 6A. That is, the opaque part 13a of this example is expanded by a predetermined dimension t in all directions as shown in FIG. 6A. The dimension t allows for the misalignment. In this case, however, there will remain an unexposed, projecting part 31b which is covered by the expanded part of the opaque part 13a of the first photomask 10 and by the elongated opaque part 25 of the second photomask 20, as shown in FIG. 6B, so that the resultant pad has a shape different from the designed shape.